0007123: QT_DEBUG define should be moved from UseQt4.cmake to FindQt4.cmake

Description

I noticed that QT_DEBUG and QT_NO_DEBUG are quite important flags.

In the case of Phonon backends, it was impossible to load a backend (== a Qt plugin) compiled with MinGW without QT_DEBUG/QT_NO_DEBUG being defined.
This issue was detected only with MinGW, it worked fine with MSVC and Linux/GCC. Once the flags QT_DEBUG/QT_NO_DEBUG were added, the problem was solved.
This typically a kind of issue very difficult to solve, who thinks about a define not there when it works using other compilers/OS?